Commercial and research laboratories have the need for the long term storage of biological materials such as blood, bone, tissue samples and biological cultures. The long term storage of such biological materials requires a temperature typically lower than about 150.degree. K. Although such biological materials are often stored at temperatures as low as that of, e.g., liquid nitrogen (77.degree. K), temperatures of about 140.degree..+-.10.degree. K. are generally suitable for such long term storage requirements.
State of the art mechanical refrigeration units can develop effective, low cost refrigeration to only about a 190.degree. K. temperature level for two stage mechanical refrigeration units and about a 170.degree. K. temperature level for three stage mechanical refrigeration units. Although multistage mechanical refrigeration units are capable of supplying refrigeration at a level of about 140.degree. K., such units are complex and expensive and therefore often undesirable for such routine storage requirements.
Refrigeration units commonly used for the storage of biological materials employ the use of a cryogenic liquid, most particularly liquid nitrogen, as the refrigeration source. Such state of the art liquid nitrogen refrigeration units typically comprise an insulated storage container holding a quantity of liquid nitrogen in the bottom of the storage area and have a shelf positioned above the liquid nitrogen level on which shelf the biological materials are stored. Such a container includes a lid in the top of the container for providing access for the insertion or removal of the biological materials. Refrigeration is provided by the evaporation of liquid nitrogen. It is apparent that such a refrigerated storage container has the disadvantage that expensive liquid nitrogen at a temperature of 77.degree. K. must be evaporated to provide the sole refrigeration source in an application where a storage temperature of only about 140.degree. K. is typically required.
Such state of the art liquid nitrogen refrigeration units have the further disadvantage in that opening the lid of the container for either insertion or removal of the biological materials significantly increases the evaporation and hence loss of the expensive liquid nitrogen refrigeration source. Such insertions or removals of the biological materials can be a common occurrence in commerical or research laboratories employing biological materials.